The present invention relates to a vibrating spool density sensor used to determine the density of a fluid in a tank, for example, an aircraft fuel tank. More particularly, the present invention relates to a vibrating spool densitometer having an inut signal which is dependent only upon the fluid in which the sensor is immersed, and is not dependent upon the length, shielding, or capacitive loading of the cable which connects the densitometer to an indicator.
Vibrating spool densitometers are known for determining the density of a fluid in a tank. Such densitometers are particularly useful in an aircraft fuel tank for providing fuel status to the pilot. Basically, a vibrating spool densitometer comprises a probe inserted into the fuel tank, and an interface for coupling the probe output signal to a processor which provides an appropriate indication to the pilot. The probe includes a vibrating vane or spool which is stimulated by a spool driver to vibrate at resonance. A pickup coil or crystal picks up vibrations from the tank and provides an output signal containing information about the density of the fluid in the tank.
In the prior art, an interface unit for the vibrating spool density sensor was provided near the fuel tank to receive the probe output signal and condition it for transmittal to a processor. However, such interface units are extremely sensitive to cable capacitance which thus precludes the use of protective shielding cable. Furthermore, the sensitivity to cable capacitance required the interface unit to be located within 20 feet of the density sensor. Typically, a separate electronic interface unit was mounted near the fuel tank containing the sensor. On a large aircraft, as many as six fuel tanks were used, thus requiring six electronic interface units. The number and location of such interface units obviously increased the aircraft weight and diminished reliability.
Such prior art densitometers are disclosed, inter alia, in U.S. Pat. No. 4,546,641 to Nguyen. Since the output of sensor 14 therein comprises two separate lead lines, preamplifier 16 must be located near the fuel tank.
Likewise, a known densitometer is disclosed in U.S. Pat. No. 4,495,818 to Ikeda et al. In FIG. 5, Ikeda discloses an electrical circuit for vibrating the vibrator 1 at its resonant frequency. Amplifier U4 is used to rotate the phase of the output signal from the limiter to satisfy the condition for self-oscillation. However, the device of Ikeda et al is directed to the physical construction of the pressure transducer, and also must include an interface unit coupled near the fuel tank.
Finally, U.S. Pat. No. 4,215,566 to Ghahramani discloses a vibration densitometer having a magnetostrictive drive with a coil and a crystal pickup. A loop circuit including an driver amplifier provides the coil with a voltage twice that ordinarily provided. However, Ghahramani also requires an interface unit coupled in proximity to the fuel tank.
Such known vibrating densitometer interface units were required to provide the oscillator with components necessary for generation of a frequency for processing by an appropriate fuel tank signal conditioner. The remote signal conditioner was required because of the characteristics of the densitometer, the interface electronics, and the interconnecting cable. Thus, known vibration densitometers require additional electronic units, thereby increasing the cost, complexity, weight, and reliability risk of the aircraft.